1. Field of the Invention
This invention relates to lighting devices and in particular to solid state lighting devices comprising one or more light emitting diodes and remote conversion materials.
2. Description of the Related Art
Light emitting diodes (LED or LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted from the active layer and from all surfaces of the LED.
In order to use an LED chip in a circuit or other like arrangement, it is known to enclose an LED chip in a package to provide environmental and/or mechanical protection, color selection, light focusing and the like. An LED package can also include electrical leads, contacts or traces for electrically connecting the LED package to an external circuit. In a typical LED package 10 illustrated in FIG. 1, a single LED chip 12 is mounted on a reflective cup 13 by means of a solder bond or conductive epoxy. One or more wire bonds 11 connect the ohmic contacts of the LED chip 12 to leads 15A and/or 15B, which may be attached to or integral with the reflective cup 13. The reflective cup may be filled with an encapsulant material 16 which may contain a wavelength conversion material such as a phosphor. Light emitted by the LED at a first wavelength may be absorbed by the phosphor, which may responsively emit light at a second wavelength. The entire assembly is then encapsulated in a clear protective resin 14, which may be molded in the shape of a lens to shape the light emitted from the LED chip 12.
LED packages 10 can generate white light by having a blue emitting LED chip 12 covered by a conversion material that absorbs blue light and re-emits yellow light. Some of the blue light passes through the conversion material without being converted such that the LED package 10 emits a white light combination of blue and yellow light. While the reflective cup 13 may direct light in an upward direction, optical losses may occur when the light is reflected (i.e. some light may be absorbed by the reflector cup due to the less than 100% reflectivity of practical reflector surfaces). In addition, heat retention may be an issue for a package such as the package 10 shown in FIG. 1, since it may be difficult to extract heat through the leads 15A, 15B. Heat from the LED chip 12 and the surrounding components can also spread into the conversion material, which can degrade the conversion material over time, changing its conversion characteristics. This in turn can result in the LED package 10 emitting different colors of light over time.
A conventional LED package 20 illustrated in FIG. 2 may be more suited for high power operations which may generate more heat. In the LED package 20, one or more LED chips 22 are mounted onto a carrier such as a printed circuit board (PCB) carrier, substrate or submount 23. One or more of the LED chips 22 can be covered by a conversion material so that the particular LED chip emits a white light combination of light from the LED chip and the conversion material. A metal reflector 24 mounted on the submount 23 surrounds the LED chip(s) 22 and reflects light emitted by the LED chips 22 away from the package 20. The reflector 24 also provides mechanical protection to the LED chips 22. One or more wirebond connections 11 are made between ohmic contacts on the LED chips 22 and electrical traces 25A, 25B on the submount 23. The mounted LED chips are then covered with an encapsulant 26, which may provide environmental and mechanical protection to the chips while also acting as a lens. The metal reflector 24 is typically attached to the carrier by means of a solder or epoxy bond.
Heat is more efficiently radiated from the LED chips 22 through the metal reflector 24, to the submount 23, and any heat sink. Heat from the LED chips, however, can still spread into the conversion material, causing degradation of the conversion characteristics. Like the package 10 described above, this can result in changing emission characteristics for the LED package over time.
U.S. Pat. No. 6,350,041 to Tarsa, entitled “High Output Radial Dispersing Lamp Using a Solid State Light Source,” discloses a number of lamp embodiments comprising one or more solid state light sources at one end of a separator, and a disperser at the other end of the separator. Light from the LED light sources travels down the separator where it can be dispersed in a radial pattern by the disperser. The disperser can also comprise a wavelength conversion material that can convert all or some of the incident light from the light sources. In different embodiments the light sources can emit blue light and the disperser comprises conversion material that absorbs blue light and re-emits yellow light. The light emitting from the disperser can comprise a white light combination of blue light from the light sources and yellow light from the conversion material. In another embodiment, an enclosure can surround the separator and disperser and can contain a yellow phosphor. Blue light from the light sources can be radially dispersed by the disperser and can pass through the enclosure where at least some of the blue light is converted to yellow. The enclosure can then emit a white light combination blue and yellow light from the phosphor.
One characteristic of conversion materials is that directional light that is incident on the conversion material that is absorbed and re-emitted by the conversion material is emitted in all directions. In embodiments where the LED chip is covered by a conversion material, some of the light re-emitted from the conversion material can be directed back toward the package where it can be absorbed. In other embodiments, the re-emitted light can be directed in a path that causes it to pass through additional conversion material where it can be absorbed. In lamps having an enclosure with a conversion material, a portion of the light can be absorbed and re-emitted back inward to the enclosure. The converted light must again pass through the enclosure before it emits from the lamp, and during these additional passes light can be absorbed by the conversion material. This absorption can reduce the overall emission efficiency of lamp.